VB.Net Len(structure) = invalid type in structure

When i try to read the memory size of my structure i get the Exception:

"File I/O of a structure with field 'cbSize' of type 'UInt16' is not valid."

My structure:

 <StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Ansi, Pack:=1)>
 Public Structure MyStructure
    Public V1 As UInt16         

    <MarshalAs(UnmanagedType.ByValTStr, SizeConst:=30)> _
    Public V2 As String 
    Public V3 As Integer             
    <MarshalAs(UnmanagedType.ByValTStr, SizeConst:=256)> _
    Public V4 As String    
    Public V5 As Byte         
    Public V6 As Byte            
    Public V7 As UInt16        
    Public V8 As UInt16         
    Public V9 As UInt16       


    Public V10 As Integer            
    Public V11 As Integer      
    <MarshalAs(UnmanagedType.ByValTStr, SizeConst:=8)> _
    Public V12 As String  
 End Structure

I wrote a function which counts the value of each Len(Vxx). This function works, but im getting the wrong value...

 size = Len(V1) + Len(V2) + Len(V3) ....

When i change the value Type to Int16 there are no problems with Len(structure).

Now my question: Why it doesn't work with UInt16?

And

Is there any way how i can use UInt16 and get the right value from Len()?

Thank you for your suggestions.

Use Marshal.SizeOf() like this:

    Dim ms As New MyStructure
    ' ... populate "ms" ...
    Dim sz As Integer = Marshal.SizeOf(ms)